1. Field of the Invention
This invention relates to balls for sports which have a reinforcing layer of a yarn wound about a rubber tube and more particularly, to a method for making such a ball as mentioned above.
2. Prior Art
Ball games using a soft volleyball, etc. as leisure sports are popular to people regardless of age or sex wherein the soft volleyball uses a rubber ball comprising a single layer rubber. However, the rubber ball of this type has following problems. The rubber ball has no reinforcing properties and is deformed or swollen when it is used for a couple of hours. If the inner pressure of the ball exceeds 0.1 kg/cm.sup.2, the rubber ball is swollen anomalously. Furthermore, if the rubber balls are accommodated in a basket, etc. in which the rubber balls are heaped up in a multiple layers, those positioned on the lowest layer receive high pressure, which causes the lack of durability thereof such as deformation or breakage thereof.
Bonded balls have been heretofore made in the following manner. Initially, a round rubber tube is endlessly wound with 3,000 to 4,000 meters of a 50 to 200 denier polyester or nylon (polyamide) filament. The thus wound tube is then covered with a rubber cover, followed by vulcanization in a mold at a temperature of 150.degree. to 180.degree. C., thereby obtaining a so-called carcass. If the mold has been preliminarily marked with a panel shape and an emboss design, the resultant carcass per se can be provided as a rubber ball. Alternatively, when a chloroprene rubber adhesive is applied to the carcass and attached with a skin material such as a natural, artificial or synthetic material, there is obtained a so-called leather ball. Although this bonded ball has good durability, there is the problem that the ball is so hard that when it hits one on the fingers or arm, one feels a pain. Thus, the ball is poorer in the touch than sewn balls.
In order to overcome the above problems, Japanese Laid-open Patent Application No. 55-103877 sets forth a ball which has a round rubber tube, a nylon filament yarn and a urethane elastomer yarn which are randomly wound about the rubber tube in combination to form a wound yarn layer. By this, the impact of the ball is absorbed. However, the proportion of the urethane elastomer yarn having good impact absorption in the total of the yarns is small. In addition, the urethane elastomer used has the same quality as a rubber cover for the ball. This merely produces the effect that the rubber cover is slightly increased in amount. Thus, impact force cannot be satisfactorily reduced. Alternatively, Japanese Laid-open Patent Application No. 56-15769 sets out a ball wherein instead of the urethane elastomer yarn used in the above application, there are used a rubber thread and a nylon filament or a combination of a nylon filament and a nylon crimped yarn, which are separately wound about the tube, thereby providing a wound yarn layer. However, this ball is not satisfactory as well with respect to the effect of reducing the impact force, like the ball of the above-mentioned application. It will be noted that the nylon-crimped yarn is one wherein a nylon filament is twisted, thermally set and then untwisted to impart twist strain thereto, or a nylon filament is exerted with a compression force along the axial direction of the filament to cause buckling and is thermally set to impart a zigzag crimp thereto. In both cases, a kind of spring-operated elastomer is imparted to the nylon-crimped yarn. In this connection, however, if the crimped yarn suffers a slight degree of tension during the fabrication or in use, by means of a plurality of guides provided in a yarn winding path, the twist strain or the zigzag crimp is lost. This results in a loss of the spring-operated elasticity, with the original behavior of the nylon filament being undesirably shown. Thus, an expected effect of reducing the impact force or a soft touch has not always been attained. In view of the above, these finished yarns are generically called non-stretchable yarn herein.
Since the impact force is mitigated, the balls in the first to third embodiments are suitable for beginners such as for soft volleyball. However, when athletes make the best use of techniques which require severe use conditions, under which the balls are unsatisfactory with respect to the durability, the balls according to the fourth to sixth embodiments meet this requirement.